lacking rigid protocol

Now that I have space to work on things (albeit not much time) and more things to work on and have recommenced active Wombling, abandoned-looking bicycles have become even more interesting. This one's rack was hanging off the back, attached only at the dropouts, looking not-very-just-locked-up-whilst-the-owner-was-at-work, unless it was a deliberate thing to make it not-immediately-rideable, even if a thief was carrying a spare wheel with them. How long must a bicycle have been sitting, forgotten or abandoned, for harvesting of remaining useful parts to be reasonably considered salvaging rather than twocking? It'd be a useful extra function of traffic wardens to monitor racks and tag bikes which have been there for a while, stating a date at which they would be considered officially abandoned (and scheduled for forcible removal/donation to the Bike Station) at which point people (proper people rather than thieves) would be welcome to descend upon them like vultures with Allen keys.